1. Field of the Invention
This invention relates to a method and apparatus for converting paraffins to lubes and other higher hydrocarbons, such as gasoline range or distillate range fuels. In particular, it relates to methods and apparatus which combine the operation of catalytic (or thermal) dehydrogenation of a paraffinic feedstock to produce olefins and the operation of a two-stage catalytic reactor system to convert olefins to gasoline and distillate boiling range materials, and downstream units to recover lubes from the distillate.
2. Discussion of the Prior Art
It has been established that the conversion of paraffins, such as propane and butane, to mono-olefins, such as propylene and butylene, can be accomplished by thermal or catalytic dehydrogenation. A general discussion of thermal dehydrogenation (i.e., steam cracking) is presented in Encyclopedia of Chemical Technology, Ed. by Kirk and Othmer, Vol. 19, 1982, Third Ed., pp. 232-235. Various processes for catalytic dehydrogenation are available in the prior art. These processes include the Houdry Catofin process of Air Products and Chemicals, Inc., Allentown, Pa., the Oleflex process of UOP, Inc., Des Moines, Ill. and a process disclosed by U.S. Pat. No. 4,191,846 to Farha, Jr. et al. The Houdry Catofin process, described in a magazine article, "Dehydrogenation Links LPG to More Octanes", Gussow et al, Oil and Gas Journal, Dec. 8, 1980, involves a fixed bed, multi-reactor catalytic process for conversion of paraffins to olefins. Typically, the process runs at low pressures of 5-30 inches of mercury absolute, and high temperatures with hot reactor effluent at 550.degree.-650.degree. C. Dehydrogenation is an endothermic reaction, so it normally requires a furnace to provide heat to a feed stream prior to feeding the feed stream into the reactors. The UOP Oleflex process, disclosed in an article "C.sub.2 /C.sub.5 Dehydrogenation Updated ", Verrow et al, Hydrocarbon Processing, April 1982, used stacked catalytic reactors. U.S. Pat. No. 4,191,846 to Farha, Jr. et al teaches the use of group VIII metal containing catalysts to promote catalytic dehydrogenation of paraffins to olefins.
Recent developments in zeolite catalysts and hydrocarbon conversion methods and apparatus have created interest in utilizing olefinic feedstocks for producing heavier hydrocarbons, such as C.sub.5.sup.+ gasoline, distillate or lubes. These developments have contributed to the development of the Mobil olefins to gasoline/distillate (MOGD) method and apparatus, and the development of the Mobil olefins to gasoline/distillate/lubes (MOGDL) method and apparatus.
In MOGD and MOGDL, olefins are catalytically converted to heavier hydrocarbons by catalytic oligomerization using an acid crystalline zeolite, such as a ZSM-5 type catalyst. Process conditions can be varied to favor the formation of either gasoline or distillate range products. In U.S. Pat. Nos. 3,960,978 and 4,021,502, Plank, Rosinski and Givens disclose conversion of C.sub.2 -C.sub.5 olefins, alone or in combination with paraffinic components, into higher hydrocarbons over a crystalline zeolite catalyst. Garwood et al have contributed improved processing techniques to the MOGD system, in U.S. Pat. Nos. 4,150,062; 4,211,640; and 4,227,992. Marsh et al, in U.S. Pat. No. 4,456,781, have also disclosed improved processing techniques for the MOGD system. U.S. Pat. No. 4,433,185 to Tabak teaches conversion of olefins in a two-stage system over a ZSM-5 or ZSM-11 zeolite catalyst to form gasoline or distillate.
Olefinic feedstocks may be obtained from various sources, including from fossil fuel processing streams, such as gas separation units, from the cracking of C.sub.2.sup.+ hydrocarbons, such as LPG (liquified petroleum gas), from coal by-products and from various synthetic fuel processing streams. U.S. Pat. No. 4,100,218 (Chen et al) teaches thermal cracking of ethane to ethylene, with subsequent conversion of ethylene to LPG and gasoline over a ZSM-5 type zeolite catalyst.
The conversion of olefins in a MOGDL system may occur in a gasoline mode and/or a distillate/lube mode. In the gasoline mode, the olefins are catalytically oligomerized at temperature ranging from 400.degree.-800.degree. F. and pressure ranging from 10-1000 psia. To avoid excessive temperatures in th exothermic reactor, the olefinic feed may be diluted. In the gasoline mode, the diluent may comprise light hydrocarbons, such as C.sub.3 -C.sub.4, from the feedstock and/or recycled from debutanized product. In the distillate/lube mode, olefins are catalytically oligomerized to distillate at temperature ranging from 350.degree.-600.degree. F. and pressure ranging from 100-3000 psig. The distillate is then upgraded by hydrotreating and separating the hydrotreated distillate to recover lubes.
Although distillate and lubes can be produced from propane and butane by the prior art, using dehydrogenation integrated with MOGDL, there are several problems with integrating these processes. For example, U.S. Pat. No. 4,413,153 (Garwood et al) discloses a system which catalytically (or thermally) dehydrogenates the paraffins to olefins, and then reacts the olefins by catalytic oligomerization (MOGDL), in a distillate/lube mode, to distillate range material which can be upgraded to lubes. Catalytic oligomerization in the distillate/lubes mode is a high (100-3000 psig) pressure process, whereas dehydrogenation is favored by low (less than 25 psig) pressure. Also, the dehydrogenation zone effluent is in vapor form. As a consequence a compressor is required for pressurizing the effluent to feed a catalytic oligomerization reactor zone operating the the distillate lube mode, thus resulting in expensive compression costs. Moreover, conversion of paraffins to olefins in dehydrogenation is slow, so dehydrogenation produces a dilute (20-50%) olefinic stream which requires expensive gas plant separation to recycle the paraffins back to a dehydrogenation reactor. The olefins should also be separated from paraffins prior to compressing and feeding the olefins to a higher pressure catalytic oligomerization reactor zone because only olefins oligomerize to form heavier hydrocarbons. Sending combined olefins and paraffins to the oligomerization reactor zone would increase compression costs and require larger reactors. Also, it is preferable to separate paraffins from olefins to factilitate recycle of paraffins to the dehydrogenation zone where they can be converted to olefins. However, a gas plant is required to separate paraffins from olefins because the dehydrogenation effluent stream comprises C.sub.4.sup.- olefins and C.sub.4.sup.- paraffins which are difficult to separate from one another.
It would be desirable to provide a method and apparatus for producing lubes from paraffins, such as propane and/or butane, which minimizes the problems of compression and gas plant costs and these are the problems to which the present invention is directed.